1. Field of the Invention
The invention relates to a molding compound. More particularly, the invention relates to a molding compound comprising a thermosetting resin, vinyl monomer, catalyst, filler and a particular rubber component.
2. Description of the Prior Art
The use of polymeric materials in concrete formulations is well known. One common composite material is called polymer-impregnated concrete wherein hardened portland cement concrete is impregnated with a liquid monomeric material which is subsequently polymerized in situ. These materials have remarkable durability and resistance against salts and the like which make their use especially attractive for concrete pipe, desalting plants, tunnel support linings and bridge decks. Another system is a polymer-portland cement concrete which is produced by adding a monomeric or polymeric material to a fresh portland cement concrete mixture which is subsequently cured and polymerized after placement. It will be appreciated that both the above systems are hydraulic systems, i.e., they require the use of water to cure the portland cement. Accordingly, another system has more recently been discovered which is called polymer concrete which contains little or no cement and no water. This is a composite formed by polymerizing a monomeric material with aggregate (gravel, sand, etc.).
The resin most often employed in present polymer concrete compositions are the so-called acrylate resins, especially, methyl methacrylate. These acrylate resins are relatively inexpensive; however, they suffer a multitude of serious shortcomings. For example, they exhibit high volatility, high toxicity, high flammability, and are explosive. More importantly, they exhibit high shrinkage which severely limits their useful life in some applications.
A number of these serious environmental shortcomings (toxicity, volatility, etc.) were eliminated by the replacement of the methacrylate resins with polyester resins in combination with an aggregate.
Compositions have been discovered which exhibit increased and improved physical and chemical properties, especially reduced shrinkage fter cure which incorporate polyesters and fillers containing fly ash. These compositions had problem with shrinkage and surface appearance.
Unsaturated polyester resins, that is, copolymerizable mixtures of (1) a polyester of an alpha, beta-ethylenically unsaturated dicarboxylic acid, optionally a saturated dicarboxylic acid, and a dihydric alcohol, and (2) an ethylenically unsaturated monomer capable of cross-linking the polyester molecules at the points of unsaturation, are extensively utilized commercially as sheet molding resins. These resins can be utilized in the production of a wide variety of shaped articles such as boats, automobile bodies, heater housings, containers of various types, toys, awnings, business machine bodies, electrical housings and the like. One particular advantage obtained by the use of unsaturated polyester resins in molding operations is the fact that little and sometimes no pressure need be used. Low pressure molding is possible because no condensation by-products that must be removed are formed during the curing of such resins.
The production and use of sheet molding compounds (SMC) is well known. Various formulations are disclosed in the following U.S. Pat. Nos.: 3,231,634; 3,674,893; 3,773,709; 3,857,812; 3,882,078; 3,989,769; 3,992,479; 3,998,909; 4,020,036; 4,096,107; 4,145,381 and 4,400,478. A typical SMC formulation contains an unsaturated polyester resin, styrene monomer, rubber, peroxide catalyst, surface agent (e.g. zinc stearate), filler (e.g. calcium carbonate), thickening agent (e.g. magnesium oxide), glass fibers and pigments.
The composition disclosed in the sheet molding compound prior art contain a thickening agent, e.g., MgOH or MgO and a rubber component to enhance physical properties and reduce shrinkage.
These SMC compounds are designed so that thickening takes place very slowly, e.g., several days, and so that they may be molded 7 to 14 days after mixing. SMC materials are compression molded and always use high pressures up to 2000 psi to fill the mold and during curing.
In the past other thermoplastic polymer modifiers used in the systems phase separated when exposed to the high temperature high pressure curing conditions. Shrink control was attributed to this phase separation in these systems.
Molding or curing temperatures range between 100.degree. C. and 200.degree. C.
What was desired was a polymer concrete composition which could be mixed, molded and cured at atmospheric pressure and ambient temperature in less than 1 to 2 hours and which would have good physical properties, good surface appearance and little or no shrinkage.